An image sensor is a semiconductor device that converts an optical image to an electrical signal. Image sensors are generally classified into charge coupled device (CCD) image sensors and complementary metal oxide semiconductor (CMOS) image sensors. A CMOS image sensor is also referred to as a CIS. The CIS includes a plurality of pixels arranged in a two-dimensional sensor array. Each of the pixels includes a photodiode (PD). The PD converts light incident thereto into an electrical signal. As semiconductor devices become increasingly integrated, image sensors are also becoming more highly integrated. Due to the high integration of image sensors, the size of each pixel is reduced, and a risk of crosstalk between pixels increases.